U.S. Pat. No. 8,878,238, assigned to the present assignee and incorporated herein by reference, describes an IGTO device using trench gates and having PNPN layers which form vertical NPN and PNP bipolar transistors. When the gate is biased sufficiently high, the beta of the NPN transistor increases due to its base being narrowed by the gate field, causing the product of the betas of the NPN and PNP transistors to be greater than one. This condition initiates the turn-on of the IGTO device. To turn the device off, the gate is grounded, increasing the base width of the NPN transistor. No negative gate voltage is needed to turn off the device. This device works well but, with high currents, latch-up may occur, preventing the device to be turned off by grounding the gate. Further, the turn on and off voltages are susceptible to variations from lot to lot.
What is needed is an IGTO device with improved turn on and turn off characteristics.